Mac mail will not download new o365 email

I have the same problem, also just started happening! I missed this thread when searching for answers and just posted basically the same question (sorry everyone). Will deleting my exchange account cause me to lose email? I have a lot of email!

I’ve had the same issue since yesterday morning. I have deleted the account on all devices and factory reset mail and still have the same issue. As stated, if I click on rebuild it does download the latest mail. Other email accounts work fine.

The same problem is duplicated on all my Macs leading me to believe it may be a Microsoft change as I don’t see any recent changes to my Mac.

I’m going to blackout my last system update and try the client again to confirm.

I have tested the Mac OS Mojave client, and it's working, but not on Catalina.

Mojave Mail client 12.4 (3445.104.11)

Catalina Mail Client 13.4 (3608.8023.2.2)

I have an admin in the office that was using Catalina and Outlook for Mac (No issues), Started up his Apple Mail, and hasn't had the experience. I have had my credentials reset, and I have moved mail from the inbox, the count is down from 40K to 6K items.

I have deleted the mail, cache, and com files to factory reset the mailbox (after credentials reset) and removed the exchange account. I'm still having the same issue on all systems.

At this point, it doesn't appear to be a Microsoft issue. Any suggestions?

It could still be Microsoft; however, my testing is pointing to something unstable in 10.15.4. As I indicated in my last post, I was able to run it on Mojave and have now created two VM's with 10.15 and 10.15.3. Both VM's are running without issues, and there are no delay in their sync.

Surprisingly enough, all three of my Macs with 10.15.4 started to receive emails last night, but I notice that the activity window would hang on connecting, then again on checking Mac for extended periods of time (up to 15 minutes). It also took long periods of time to move files around. As of this evening, two have stopped getting new mail, and one does, but there is about a 5-10 minute delay from the VM's. From my perspective, this is a problem with 15.4.

Since 15.5 went into beta five early this week, I'm expecting a public release within the next week or two. So I'm not sure how much time I want to put in to open a ticket with Apple until I see if 15.5 fixes the issue.

I updated two of my three machines with 10.15.5 yesterday, and the problem appears to be resolved. The third machine is still on 10.15.4 and continues to have the problem. Plan to upgrade that machine tomorrow.

Try deleting the account from your Mac and add it back.

Having exactly the same issue On Catalina and across machines. Only way to get new mail for O365 account is to hit rebuild.

Update: 10.15.5 did not resolve my issue. It started up again today. I have open a ticket with apple support who is reviewing my system. I will update on their findings.